In the case of a publicly known conventional water-based ballpoint pen including an ink tank filled with low-viscosity ink directly poured therein, air needs to be taken into the ink tank for discharge of the ink during writing. If temperature increases or atmospheric pressure drops rapidly during writing, or if pressure inside the tank increases as a result of temperature increase or drop of atmospheric pressure while a cap is attached and then the cap is removed, the ink may jet out of the tank to soil a sheet of paper being written on or clothes, for example. Hence, action against the jet of ink has been taken by employing a comb-like ink guidance member called a collector or by employing a structure for buffering pressure inside the ink tank. However, these methods increase a number of parts, disadvantageously causing cost increase or complicated assembly process.
Patent literatures listed below disclose techniques of solving the aforementioned problem. According to these techniques, an ink tank is made of a flexible material and a valve (or a spring) is installed on a writing tip portion, thereby making the ink tank shrink in response to writing.
Patent Document 1: JP 2000-177290 A.
Patent Document 2: JP S29-13711 Y.
Patent Document 3: JP S35-25220 Y.